Adolescent cannabis use and the later onset of bipolar disorder: protocol for a systematic review and meta‐analysis of prospective cohort studies

BACKGROUND: Cannabis is used by adolescents worldwide. Adolescents are more susceptible to the psychological effects of cannabis because their brains are still developing. Cannabis use in adolescents has been reportedly associated with later onset of bipolar disorder. AIMS: The purpose of this study is to systematically review and analyze longitudinal prospective cohort studies of cannabis use during adolescence and evaluate the risk of developing bipolar disorder. METHODS: We defined the participants, exposures, comparisons, and outcomes (PECO) as follows: (P) adolescents in the 10–19‐year age group at the baseline survey; (E) cannabis use at least once during lifetime; (C) never‐used cannabis over lifetime; and (O) the onset of bipolar disorder. A systematic search for published prospective cohort studies will be conducted by using the following electronic databases: PubMed, EMBASE, PsycINFO, and Japan Medical Abstracts Society. The quality assessment will be performed by using Risk Of Bias In Non‐randomized Studies – of Interventions. Meta‐analysis will be done if the included studies that exist are more than three. Heterogeneity will be assessed using Cochran's Q test and I(2). Funnel plots and Egger's test will be done to assess publication bias. DISCUSSION: This study will clarify the association between adolescent cannabis use and the subsequent development of bipolar disorder, which could be useful for future research directions and policy making.